Multiple phase motors with bipolar drive according to the present invention comprise stepper motors on the one hand, such as bipolar stepper motors, either permanent magnet stepper motors or not, as long as forced current can exist, and brushless sensorless DC motors on the other hand.
Stepper motors are widely used in positioning applications and robotics in order to reach high accuracy without an external sensing element. Examples of such positioning applications may be e.g. driving of flaps, belts, mechanical heads etc. The absence of an external sensing element reduces system cost, yet it implies open-loop control of the positioning. This open-loop control, however, can have a negative impact on the system's quality:                Lost steps are not noticed by the controlling part of the application, which can imply poor positioning accuracy.        A stall condition causes additional noise.        A stall condition causes extra wear on the mechanical components attached to the rotor of the stepper motor.        Speed-variations are not visible and make closed-loop speed control impossible.        
It is possible to reduce the negative effects of the open-loop control by implementing a so-called stall-detection or, more generally, a rotor-speed sensing capability.
One possibility of stall-detection for stepper motors is described in EP-A2-0046722. The actual movement of the stepper motor rotor in response to the energizing of the motor stator windings by excitation signals presented in each step interval of the motor is detected. This is done by measuring the amplitude of the voltage signal induced in a non-energized stator winding as a result of the presentation of the excitation signals to the energized stator windings, both in a present step interval and in an immediately preceding step interval. The physics behind the stall detector in the above document is related to the operation of a transformer: a primary coil (active motor winding) generates a magnetic flux which generates in a secondary coil (inactive motor winding) an induced voltage. In case the motor is able to rotate, the magnetic coupling between the coils is small, and there is a small residual magnetic field energy. In case, however, the rotor is blocked, the residual magnetic field energy is larger, hence the secondary coil shows an increased induced voltage. An induced voltage amplitude which exceeds a threshold indicates a failure of the rotor to respond to the newly energized stator windings and may be used as an indication of failure in the motor. This known device works on the principle of current/voltage signals appearing on a non-energized coil (at the beginning of the non-energized phase) as a result of energizing another coil. In as much as the first decay pulse has died out, or on top of the remaining signal, the back emf (for higher rotation speeds) is measured. The principle described is sensitive to supply voltage, because the amplitude of the primary coil varies with supply voltage.
Brushless sensorless DC motors (BLSL-DC) are used in various speed-controlled applications like fans, pumps, ventilator motors for PCs etc. Because of price and manufacturing cost limitations, the motors are not equipped with sensors (like Hall sensors) to monitor rotor speed. Most of the low-cost applications do not require speed detection. However for a higher quality operation, it is highly desired to have speed indication. Some topologies of BLSL-DC motors with star-connected coils allow relatively easy monitoring of BEMF signals (see FIG. 11, 3-phase motor). A non-activated coil picks up a moving magnetic field of the rotor 2, and a bemf voltage signal is visible across the non-activated coil terminals. A wide range of implementations have been described. However, in the case of coils that are not connected in a star, the speed-monitoring or stall-detection is difficult.